PCT patent application no. PCT/EP04/006723 describes a method of controlling in-can pressure during thermal processing by bonding a diaphragm or “foil” panel of lidding material such as aluminium/polymer laminate or aluminium coated with lacquer to an annular component. The thickness of the aluminium in the material of this panel is at least 60 microns, preferably about 70 to 80 microns. In the method of PCT/EP04/006723 the panel is stretched and the annular component and panel bonded thereto are fixed to a filled can. During thermal processing the closure panel assumes a generally dome shaped profile so as to provide an increase in can volume.
In this type of closure, the annular component or ring includes a substantially flat sealing surface extending radially inwardly from a wall and the foil panel is fixed to the sealing surface so that the panel is substantially in the same plane as the sealing surface. The sealing surface is then reformed to an angle of 15° to 25° relative to the transverse plane of the closure and the foil panel is profiled to a domed or beaded shape. The provision of a domed or beaded shape reduces the pressure difference experienced by the panel due to the volume increase this profile gives.